US20080063007A1 - System for Communicating Data in xDSL Using Data Retransmission - Google Patents

Abstract

An apparatus for transmitting data in an xDSL system is disclosed. In an exemplary embodiment, the apparatus comprises a transmitter that sends data over an xDSL system in the form of a data transmission unit (DTU), the transmitter having physical media specific—transmission convergence (PMS-TC) communication sublayer. The apparatus may further include a retransmission unit that is implemented in the PMS-TC sublayer. The retransmission unit may include a retransmission buffer that stores and indexes transmitted DTUs in retransmit containers. The retransmit container may be defined as a time slot corresponding to a sent DTU, wherein the retransmission unit is configured to respond to a retransmission request indicating which stored DTU should be retransmitted. The stored DTU to be retransmitted may be identified by its corresponding retransmit container.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application claims benefit of U.S. Provisional Application No. 60/907,833, filed Apr. 18, 2007, and U.S. Provisional Application No. 60/825,542, filed Sep. 13, 2006, all of which are incorporated by reference herein in its entirety.
FIELD OF THE INVENTION
• Certain embodiments of the present invention relate to data communication. More specifically, certain embodiments of the invention relate to a method and system for communicating data in xDSL using data retransmission.
BACKGROUND OF THE INVENTION
• As early as the first ADSL1 standard, recovery of transmission errors in xDSL relied upon using error correction codes such as, for example, Reed Solomon (RS), together with interleaving. In addition to providing impulse noise correction, the use of RS code provided extra coding gain, therefore improving the achievable data rate of the DSL system.
• The evolution to ADSL2+ and VDSL2 standards did not question this strategy to fight impulse noise. However, the high data rate achieved together with extended impulse noise requirements forces the use of very small RS code words having many RS parity bytes. Accordingly, the RS net coding gain becomes highly negative, causing degradation in achievable bit rate.
• Current DSL systems provide Impulse Noise Protection (INP) by means of Reed Solomon FEC associated with interleaving techniques. When a high INP is requested together with a small delay constraint (or limited available interleaving memory) this technique has some drawbacks such as the RS code words introducing a lot of overhead and consequently, the high INP protection is provided at a cost reduced bit rate (RS coding gain becomes negative), and if the system cannot correct the error, a lot of user data is impacted. Further, at low noise margins, the RS decoder capability is already stressed to correct residual stationary errors, thereby preventing the RS decoder from being fully available to correct impulse noise. It can be shown that with typical xDSL and RS decoder and interleaver settings, the impulse noise correction capability is practically nonexistent below approximately a 2 dB noise margin.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
• FIGS. 1A-C are block diagrams of an exemplary transmitter.
• FIG. 2 is a block diagram of an exemplary retransmit unit and block interleaver operation.
• FIGS. 3A and 3B are block diagrams of an exemplary retransmission schemes.
• FIG. 4 illustrates a basic transmit mechanism.
• FIG. 5 is a flow chart illustrating an exemplary transmitter method.
• FIG. 6 is a flow chart illustrating an exemplary receiver method.
• FIG. 7 illustrates the performances of an interleaver scheme.
• FIG. 8 is a comparison of guaranteed rate between retransmission and interleaver schemes.
• FIG. 9 illustrates the ratio between the delays for retransmission and for interleaving.
• FIG. 10 illustrates the memory required for the interleaver and retransmission normalized by R_line.
DETAILED DESCRIPTION OF THE INVENTION
• Standard xDSL systems provide in their configuration management information base (“MIB”) a parameter called minINP (minimum Impulse Noise Protection). MinINP defines the impulse noise length that the communication link should be able to sustain without error. The configuration MIB also defines a maximum transmission delay, maxDelay. Since a (N,R) Reed-Solomon (RS) code can correct up to R/2 errors, and since the interleaver spreads a codeword over at most maxDelay ms, standard xDSL systems can use an overhead rate equal to at least: (R/2)/N>=minINP/maxDelay, for example, with a typical value of minINP (500 us) and maxDelay (4 ms), R/N>25%.
• A system that can achieve the requested minINP and maxDelay constraints while achieving a rate close to what it can achieve without impulse noise protection, i.e., without paying the associated high Reed-Solomon overhead rate cost, may use something other than RS code to achieve its impulse noise immunity. In accordance with the following disclosure, providing an error-free communication link in presence of an impulse noise of length almost equal to the maximum link delay may be achievable using the disclosed data retransmission scheme. While using retransmission may introduce jitter, an embodiment of the present invention may limit the introduced jitter in retransmission.
• The following disclosure describes a method for handling data retransmission in an xDSL system. The method uses data retransmission while limiting as much as possible the modifications needed on existing systems. The method may be implemented, for instance, on existing xDSL CO (central office) and CPE (Customer Premises Equipment) chipset. The main principles of data retransmission are enabled when the CO keeps a copy of all data blocks it sends downstream, typically for period of 5 to 8 milliseconds. When the CPE detects a corrupted data block, it asks the CO to retransmit the concerned block.
• In an embodiment, retransmission may provide an INP of at least 8 symbols, and it may allow the CPE to maximize the coding gain it can get from the usage of trellis combined with RS forward error correction (FEC). This is because RS overhead does not need to provide the INP, but can be solely selected to maximize the coding gain. Therefore, a minimal interleaving may be used to scatter the trellis errors prior to RS correction.
• FIG. 1A is a block diagram of a portion of anexemplary transmitter100. Three separate sublayers are illustrated as they may appear in an xDSL modem. Thefirst sublayer150 is the Transport Protocol Specific—Transmission Convergence (“TPS-TC) layer, thesecond sublayer152 is the Physical Media Specific—Transmission Convergence (“PMS-TC”) sublayer, and thethird layer154 is the Physical Media Dependent (“PMD”) sublayer. One skilled in the art of xDSL communications will recognize the basic functions of these three sublayers and determine where they fit in a typical OSI-based communication stack. Further, xDSL transmitters and receivers having such sublayers for both CO and CPE applications are known in the art. Such examples include Broadcom Corporation's BCM6410/6420 BladeRunner™ ADSL2+ Central Office Chipset and BCM6348 Single-Chip ADSL2+ Customer Premise Equipment Chip.
• In the configuration illustrated inFIG. 1A, the data retransmission system and method described herein may be implemented in the PMS-TC layer152 as a new functional unit—i.e., asretransmit unit104. One advantage to this approach is its implementation simplicity. Placing theretransmit unit104 in the PMS-TC layer puts it closer to the PMD layer, which is where most errors take place. Further, such placement offers greater transparency with respect to existing performance monitoring functions. This specific embodiment is described in more detail herein.
• In alternate configurations illustrated inFIGS. 1B-C, a system and method could be defined that is compatible with placement of theretransmission unit104 in different location. For instance, as illustrated inFIG. 1B, a retransmission transmission convergence (RTX-TC)layer151 could be defined between the TPS-TC150 and PMS-TC152 sublayers without departing from the scope of the present disclosure. One advantage of this approach is that it leaves the existing frame structure unchanged. In yet another configuration illustrated inFIG. 1C, a system and method could be defined that is compatible with placement of the RTX-TC sublayer151 between the PMS-TC152 and PMD154 sublayers. Most principles described herein are valid and applicable for these alternate solutions, and one of skill in the art would be able to modify the principles described herein accordingly.
• According to the disclosed retransmission principles, a data block or data transmission unit (DTU) is defined. The previously transmitted DTUs may be temporarily stored at the CO. When it is determined that a DTU was corrupted during transmission, it may be retransmitted. One skilled in the art will recognize that the current xDSL standards define at least three types of TPS-TC (Transport Protocol Specific Transmission Convergence) functions: synchronous transfer mode (STM), asynchronous transfer mode (ATM), which is typically used on ADSL systems, and packet transfer mode (PTM), which is an ethernet and generic packet interface, which is typically used on VDSL systems. A data block or DTU may thus be defined as one or several RS code words, or something different such as a block of ATM cells, PTM cells or fragments, or 65 byte packets, for example.
• The CO keeps a copy of all DTUs that it sends in downstream for a period of time. In an exemplary embodiment, the DTUs are kept for 5 milliseconds. The period of time should be sufficient to determine whether any of the sent DTUs were corrupted during transmission, and to request retransmission thereof. If the CPE detects a corrupted data block, it may send a signal to the CO requesting a retransmission of the corrupted data block.
• The size of the data block or DTU may be defined as n=1/S code words, where the value of 1/S may be an integer. This causes the data block or DTU to exactly match the size of one discrete multi-tone (DMT) symbol. In case of a single corrupted DMT symbol, for example, this scheme may have the benefit of corrupting only one data block. However, this simplification is optional. If this simplification is adopted, the repeated data block may become a repeated DMT symbol. To overcome the possibility of having the same crest factor as a result of the repeated DMT symbol, a new scrambler or some other type of data randomization scheme may be utilized, for example.
• Further simplification may be achieved by replacing theconvolutional interleaver102 by a block interleaver202 (as illustrated inFIG. 2) over D symbols, i.e. an intra codeword interleaving scheme that may not cost any interleaving memory and does not introduce any extra transmission delay.
• A retransmit container may be further defined to identify which corrupted DTUs need to be retransmitted, irrespective of the manner or degree to which the DUT was corrupted. In the disclosed retransmission scheme, the retransmit container is defined as a “data slot” or a “time slot” corresponding to a DTU. The retransmission container may be maintained at both the transmitter (Tx) and receiver (Rx) sides of the xDSL system. As long as the xDSL data rate is substantially identical at the Tx and Rx sides, the retransmit container index will be correct and synchronous at both sides. This is true even in presence of massive transmission errors.
• The retransmit container index may thus be used to reliably identify the data blocks or DTUs that needs to be retransmitted. The data blocks or DTUs corresponding to the retransmit container may comprise n RS codewords. The parameter n may be nominally chosen close to, but lower than or equal to 1/S such that the corruption of one discrete multi-tone (DMT) symbol may only call for retransmission of 1 or 2 DTUs. Two consecutive containers may not contain two consecutive data blocks, such as in the case of retransmission where a container will contain a data block which has already been transmitted.
• Every data block may also be uniquely identified by a sequence identification number (SID)131. TheSID131 information may be repeated over n bytes, 1 byte being inserted inside each codeword, as the first byte of each codeword. TheSIDs131 may increment sequentially modulo a max count, significantly larger than the depth of the retransmission queue. Amultiplexer130 may be incorporated into PMS-TC layer152 to handle addition of aSID131 to the data blocks. TheSID131 may be added after the scrambler block120, but before the forward error correction (FEC) block122, as illustrated inFIG. 1. The data block or DTU may comprise a set of data bytes grouped together and identified by the SID. An exemplary data block orDTU206 is illustrated inFIG. 2. TheSID131 increases with each new data block generated and is used to identify a new data block from a retransmitted one.
• Referring again toFIG. 1,transmitter100 may also comprise aconvolutional interleaver102.Interleaver102 receives as input one (or several fractions of one) RS codeword and produces L0 bits per symbol. In a standard transmitter, at each symbol, the interleaver expects L0 bits from coming out of the forward error correction (FEC) block122.FEC122 could be, for example, a Reed-Solomon (RS) encoder.
• Aretransmission unit104 handles the retransmission request. Theretransmit unit104 may also store and manage retransmit containers and the retransmit container index, which are used to identify transmitted and stored DTUs. It should be clear to one of skill in the art that retransmitunit104 could alternatively be placed after theFEC122, e.g., after a RS encoder. This functionally equivalent embodiment (not illustrated) highlights the fact that the retransmit queue204 (illustrated inFIG. 2) could also store RS parity bytes. Such an embodiment could increase memory constraints but reduce computational load. Operation ofretransmit unit104 is described more fully below.
• FIG. 2 is a block diagram of anexemplary retransmit unit104.Retransmit unit104 includes a multiplexer for receiving incoming data frommultiplexer130 and aretransmission request214. Further,multiplexer230 receives the data block or DTU to be retransmitted235 from aretransmission buffer204. As noted above, the DTU to be retransmitted235 is identified by its retransmission container.
• FIG. 2 also illustrates the operation ofblock interleaver202. When operating in an exemplary block repetition mode, certain settings may be applied to the frame structure of exemplary data blocks206. One setting may be to group D RS codewords together to form data block206. Supported values for D may be, for example, 1, 2, 4 and 8 or more. The values Nfec and D may be selected by a receiver (not illustrated) such that the data block size is lower or equal to the discrete multi-tone (DMT) symbol size (L/8 bytes). These ranges are exemplary and not restrictive.
• Another setting may be to include an overhead byte, theSID byte131, in each RS codewords, prior to the first data byte. TheSID byte131 may provide the data block sequence ID, and may be, for example, an 8-bit wrap around counter starting at 0 and incremented each time a new data block206 is generated. Thesame SID byte131 may be included in each RS codeword part of the same data block.
• Another setting may be to store the data block or DTU inside a retransmit unit. The retransmit unit output is a retransmit container (described above) that contains either the last data block it has received, or one of the previous data blocks it had stored. The output data block may be cyclically rotated inside the retransmit container by a byte offset equal to the retransmit container counter modulo256. The retransmit container counter is incremented each time a new retransmit container is sent. Such cyclic rotation is optional, and may occur when the DTU size matches exactly the DMT symbol size.
• Another setting may be replacing theconvolutional interleaver102 by ablock interleaver202, acting over one DTU or data block. The interleaving depth D may be set equal to the number of RS CW inside a DTU. Byte Bj with j=0 . . . Nfec*D−1 may be output by the interleaver at index k=mod(j,Nfec)*D+floor(j/Nfec).
• Yet another setting may be to use a “retransmit control channel”110 to send and receive retransmit requests. Theretransmit control channel110 may be, for example, a fixed rate channel of 3 bytes/symbols, for example, multiplexed at the gamma interface as an implicit extra latency path LP_i+1=LP_RCCwith L_RCC=24, pre-pended before the L1+L0 bytes of the data paths. Theretransmit control channel110 may be present in one direction if the block retransmission mode is enabled in the reverse direction, as illustrated inFIG. 1. In one embodiment using ADSL, a channel of two bytes per symbol may be used. In another embodiment using VDSL, three bytes may be used.
• The number of FEC parity bytes R and codeword length N may be selected such as to insure a reliable detection of uncorrectable codewords by the RS decoder. For instance, with R=16 and N<=232 it can be computed that uncorrectable codewords will go undetected with a probability lower than 10⁻⁵. This effectively may be translated to 1 error out of 10⁵could go undetected and assuming 10 errors per seconds, this is one undetected error every 2 hours. Lower residual undetected uncorrected codeword rate may be achieved by either increasing R, lowering N, or by only allowing the RS decoder to correct less than R/2 errors. The parameters R and D may be selected such that a trellis decoder error burst can be corrected. Since such a burst may typically span 10 tones, a correction capability larger than (typically) 10*12 bits/tone=15 bytes may be utilized. For example, with D=4, R>=8. Alternatively, the RS code may be selected such as to only provide error detection. In this case, which may be of interest at a low bit rate, R=2 or 4 may be selected.
• Based on the information received from the customer premise equipment (CPE), theretransmit unit104 may decide whether it should take a new data block from the RS encoder or whether it should retransmit a data block from itsown retransmission buffer204. Note that if retransmission is applied in the downstream direction, as described here, the receiver is associated with the CPE and the transmitter with the central office (CO). The reverse convention would be used if retransmission is applied in the upstream direction.
• For retransmission in the downstream direction, retransmitunit104 keeps track of which data block206 has been sent in which retransmit container using a retransmit index. A request to retransmit a given container may be accepted once by thetransmitter100. However a same data block or DTU may be retransmitted several times inside different retransmission containers. Thetransmitter100 keeps track of the time at which it has sent a given DTU or data block for the first time, and will not accept retransmission of a retransmission container that contains a data block that is older than a given threshold. This threshold may be configured as being equal to the MIB maximum delay parameter.
• At the receiving end, a receiver may de-interleave the transmitted data, and verify and correct the RS codewords present in the data block transported by the retransmit container. The receiver may also verify the correctness of the SID present in the retransmit container. At that point, the receiver may have information regarding whether one of the codewords may be uncorrectable and whether the SID may have been corrupted during the transmission. Based on these two indications, the receiver may decide what to do with the received RS codeword. Some possible actions that the receiver may take are: drop the data block; send a retransmit request to the CO for the container just received; replace a data block already present in the receive buffer FIFO304 (seeFIGS. 3A and 3B), by a newly arrived one; and push the data block in the receivebuffer FIFO304 and simultaneously take a data block out of the receivebuffer FIFO304 and continue the normal deframing processing with the RS codewords present in that data block.
• The receivebuffer FIFO304 may be a FIFO of m received data blocks, and should be long enough to allow reception of a retransmission before the data block leaves the FIFO. For each data block or DTU present in the receivebuffer FIFO304, the FIFO may be capable of determining whether it contains errors or not. At initialization time, the receive buffer FIFO may be full of correct dummy data blocks. The deframer (not shown) may therefore not use the first m data blocks that go out of the receivebuffer FIFO304.
Retransmit Request
• Any retransmission scheme relies on a feedback from a receiver. This feedback must be highly reliable. In an embodiment, the transmitter receives aretransmission request214 from the receiver. Retransmission requests214 could have different characteristics. For example, it is desirable to have redundancy in the request. As such, notice that a DTU has to be retransmitted is preferably contained in multiple requests such that if some requests get lost, there is still a possibility to receive the retransmission notice. Further, theretransmission request214 preferably requires as small a bitrate as possible in order to minimize overhead (e.g., 2 or 3 bytes per symbol in various embodiments). Therefore, the format should be dense and the request should contain as much information as possible. In other words, best use should be made of available overhead. Still further, the information contained inretransmission request214 should be understandable and meaningful independently of the history. In other words, the retransmission request is preferably self meaningful and not dependent on any previously transmitted request. Finally, theretransmission request214 should be protected by some kind of error check, such as a check sum, so that the receiver can discriminate between correct and erroneous requests with a high reliability.
• When sending aretransmission request214, the retransmission signal may indicate the retransmit container ID, the last container ID to be retransmitted, and the number of containers to be retransmitted after that container. The signal may also comprise the last received container ID, and a bitmap indicating which container need to be retransmitted. For example, if the last container ID is Cid, bit I of the bitmap then is set to 1 if Cid-I needs to be retransmitted.
• Exemplary data structures for an uncompressed retransmission request format could include {FCS, statusBitmap, lastContainerIdx}. For example:
• lastContainerIdx: this provides the ‘time stamp’ of the receiver. Using this field the transmitter knows what range of its retransmit FIFO queue has already been received and is concerned by the statusBitmap. In an embodiment, the lastContainerldx only contains the few LSB's of the index. Indeed, the MSB can easily be reconstructed by the transmitter, knowing that the roundtrip delay is approximately constant.
• statusBitmap: one bit for each of the last n received container. LSB carry the status of the container lastContainerldx, bit i carry the status of container lastContainerIdx-i. Alternatively, instead of carrying 1 bit per container, the information can be compressed in several different ways. One way is to count the trailing zeros of the uncompressed status, i.e. sent the number of consecutive wrong containers received, starting from lastContainerldx and counting backward.
• FCS: checksum to validate the correctness of the request
• Upon receiving data, the DTU or data block may be checked for errors and different actions may be taken based on whether or not errors are present in the data block. If there is an unrecoverable error in one of the RS codeword or in the received SID, the data block may be pushed as being the next expected data block (tail of the FIFO), and the data block may be marked in the queue as being erroneous. If the receivebuffer FIFO304 contains at least one correct data block, a retransmit request may be sent to the CO. If, however, the receivebuffer FIFO304 only contains wrong data blocks, there may be high chance that a retransmission may not occur on time, and consequently, the retransmission may be disabled until the receivebuffer FIFO304 is again partially filled with correct DTUs. Upon examining the data, it may be determined that there are no residual errors in DTU. In this case, the SID is equal the next expected SID. Consequently, the data block may be pushed in the receivebuffer FIFO304 and marked as being correct.
• In another situation, upon examining the data, it may be determined that the SID may not be the expected one and retransmission may be needed. If the SID does not match an index in the receivebuffer FIFO304, the DTU may be dropped if there is no correct data block in the FIFO. This may effectively result in recovering a potentially lost synchronization in case of long period with errors. The received codeword may be dropped until synchronization is achieved again. When repetition is handled at the TPS-TC level, resynchronization may not be needed and the received data block may be immediately pushed inside the receivebuffer FIFO304. However, if there are some correct data blocks in the FIFO, the received data block may be pushed at the beginning of the queue, and marked as being wrong, without asking for retransmit.
• Alternatively, if it is determined that the SID is not be the expected one and retransmission may be needed. If a correct data block is already present in the FIFO at the location corresponding the to the received data block, it may be concluded that a data block for which retransmission was not requested may have been received. This data block may be pushed at the beginning of the FIFO and marked as being wrong. Still further, if it is determined that the SID may not be the expected one and retransmission may be needed, if the data block present in the FIFO at the location corresponding to the received data block is marked as being wrong, it may be replaced and marked as correct.
• In embodiments, support for the retransmission scheme in addition to the standard interleaving mode may be negotiated during handshake in the communication system. Once established, in the CLR and CL messages, the CPE and CO may respectively announce support for this mode in the Rx direction, support for this mode in the Tx direction, their own worst case half-way roundtrip delay, and the maximum size of the repetition FIFO at the transmitter side.
• TheRetransmission Control Channel110 illustrated inFIG. 1 may carry one retransmission request every symbol. The format of the retransmission request may be as follows: a portion of the most recent retransmit container index that has been received (for example the 4 LSBs); the number of containers without errors received since the last container without errors; the number of retransmit containers that must be repeated; and a message checksum.
• Further embodiments and principles of the above described transmission scheme are be further illustrated in the diagrams ofFIGS. 3A and 3B. The general principle for an end-to-end retransmission scheme is depicted inFIG. 3A. All the transmitted DTUs are stored in aretransmission buffer204 at the transmitter side. Upon reception of a DTU, its frame check sequence (FCS) is checked and a retransmission request is immediately launched if it is found corrupted. Even if corrupted, the DTU is pushed in the receivebuffer304. If the retransmitted DTU arrives while the corrupted one is still present in the receivebuffer304, the corrupted one is replaced. If the retransmitted data unit doesn't arrive on time, the corrupted data will be further processed by the receiver data path.
• In more detail, the basic transmit mechanism is as follows (W_retdenotes the retransmission window size in DTU). If no retransmission is pending, new data bytes are stored in a new DTU, and the DTU is transmitted over line as well as stored in theretransmission buffer204. If, however, a retransmission is requested, two cases are possible. In one case, the first transmission of the requested DTU took place less than W_ret*T_DTUsecond before the current time. In that case, the DTU is retransmitted. In a second case, the first transmission of the requested DTU took place more than W_ret*T_DTUsecond before the current time. In that case, the request is discarded and a new DTU is transmitted.
• Two examples of this mechanism are depicted inFIG. 4. In both cases W_retis equal to 8 DTUS. In the upper example, the round trip delay is equal to 4 T_DTU. TheDTU #4 is corrupted a first time, and a retransmit is requested. As the first retransmit is requested 4 T_DTUafter the first transmission, the retransmission occurs. Thesame DTU #4 is again corrupted and a second request is sent back. The second request is received less (or equal) than 8 DTUs before the first request, so a second retransmit occurs and theDTU #4 is received with a delay of 8 DTUS.
• In the lower example ofFIG. 4, also two transmissions ofDTU #4 are corrupted but this time the round trip delay is 5.T_DTU. As the second request arrives more than 8 DTUs after the first transmission, the request is discarded and theDTU #4 is never transmitted, i.e. an error happens on the line. As the receiver knows that after 8 DTU, there is no chance to receive theDTU #4, theDTU #5 will be blocked in the rescheduling queue no more than 8 T_DTU.
• An alternate end-to-end retransmission scheme is described inFIG. 3B. Therein arate adaptation FIFO306 has been added to model the network processor at the input of the digital subscriber line physical layer. Assuming that the transmission rate of the DTU (equivalent to the net bearer rate) is R_dand the rate of the guaranteed service is R_in, the exemplary principle is as follows: every transmission of a new DTU, R_in.T_DTUbits enter therate adaptation FIFO306 and at most R_d.T_DTUbits leave therate adaptation FIFO306. If therate adaptation FIFO306 contains less than R_d.T_DTUbits, the missing bits, as the DTU always contains R_d.T_DTUbits, are added by the rate adaptation process of the TPS-TC, through idle cell insertion in ATM or idle bytes insertion in PTM. Note that this process is statistical and follows the rate adaptation rules of the ATM or PTM TPS-TC. In the described embodiment, every retransmit of a DTU, R_in.T_DTUbits enter the rate adaptation FIFO and no bits leave therate adaptation FIFO306.
• As noted above, in some embodiments, the retransmission may be controlled between TPS-TC and PMS-TC. As a new ‘retransmission layer,’ the data block may be defined as a group of ATM cells, PTM cells or fragments, or 65 bytes packet, for example. This approach leaves the existing frame structure unchanged. Additionally, using this approach may call for providing a new protection scheme on the data blocks, i.e., new CRC.
Exemplary Methods
• A method for communicatingdata500 is described inFIG. 5. Themethod500 illustrated inFIG. 5 is performed from the standpoint of the transmitter, which could be located, for example, at a central office facility. According to step505, a data transmission unit (DTU) is defined. In an embodiment, the DTU is sent in an xDSL data stream. As described above, a data block or DTU may defined as one or several RS code words, or something different such as a block of ATM cells, PTM cells or fragments, or 65 bytes packets, for example.
• According to step510, retransmit container is further defined as a time slot or data slot that corresponds to one sent DTU. Retransmission containers may be identified using an always-increasing index, starting from 0 at initialization time. As illustrated instep515, the retransmission container index is maintained at the transmitter along with corresponding copies of the sent DTUs and possibly with a time stamp identifying the moment of the first transmission of that DTU. As long as the xDSL data rate is substantially identical at the Tx and Rx sides, the retransmit container index will be correct and synchronous at both sides. This is true even in presence of massive transmission errors. The retransmit container index may thus be used to reliably identify the data block or DTU that needs to be retransmitted.
• According to step520, the DTUs are transmitted in an xDSL data stream. The transmitter then determines, according tostep525, whether a transmitted DTU was corrupted during transmission. There are at least two ways by which a transmitter may determine whether a DTU was corrupted during transmission. In one embodiment, a retransmission request may be received requesting retransmission of an uncorrupted copy of the corrupted DTU. In such an embodiment, according tostep530, the uncorrupted copy of the DTU is identified in the retransmission request by its corresponding retransmit container. The retransmission request may be received in the reverse direction over a retransmit control channel, and the uncorrupted copy of the DTU may be returned instead of a new one in the downstream (in case of a CO transmitter) direction.
• In an alternate embodiment, an acknowledgement may be expected for each validly transmitted DTU. If an acknowledgement is not received, it may be assumed that the DTU was not validly received and may have been corrupted. In that case, the transmitter may generate its own retransmission request. Again, according tostep530, the retransmission request may identify the unacknowledged DTU by its corresponding retransmit container. In yet another embodiment, both methods may be used together to enhance reliability that a corrupted DTU will be identified such that it may be retransmitted, and to optimize the delay before which the retransmission takes place.
• Once the retransmission request is processed, and the corresponding DTU has been identified by its retransmit container, the elapsed time since the first transmission of this DTU is verified according tostep532. In an embodiment, if the elapsed time falls within the allowed retransmit window, an uncorrupted copy of the DTU is retransmitted according tostep535.
• A method for communicatingdata600 is described inFIG. 6. Themethod600 illustrated inFIG. 6 is performed from the standpoint of the receiver, which could be located, for example, at the customer premise equipment (CPE). According to step605, an xDSL data stream having defined data transmission units (DTUs) are received from a transmitter within retransmit containers. The retransmit container is defined as a time slot that corresponds to one received DTU. According to step610, an index of the retransmit containers is maintained.
• Next, according tostep615, the receiver determines whether a received DTU was corrupted. Various error checking schemes are well known in the art. According to step620, for each corrupted DTU, a retransmission request is transmitted that requests retransmission of an uncorrupted copy of the corrupted DTU, or alternatively for each received DTU, a positive or negative acknowledgement is transmitted that lets the transmitter know which DTU needs to be retransmitted. The uncorrupted copy of the DTU is identified in the retransmission request by its corresponding retransmit container
• The retransmission request will be processed at the corresponding transmitter, and the receiver will then receive from the transmitter a retransmitted copy of the uncorrupted DTU, according tostep625. As afinal step630, the corrupted DTU substituted with the uncorrupted DTU in the xDSL data stream. As described above, a retransmission buffer is used for this purpose.
Control Parameters
• Based on the retransmission model with rate adaptation queue, we can derive three high level parameters:
• • The delay in ms equal to (N_ret+W_ret).T_DTU. Note that the delay may be split in the delay due to the resequencing of the retransmissions, W_ret.T_DTUand the delay due to the retransmissions of the DTU, N_ret.T_DTU.
  • The impulse noise protection (INP) equal to N_ret.T_DTU/T_sin DMT symbol with T_sthe symbol duration (e.g. 250 us).
  • The supported minimum interarrival (IA) between two worst case impulses, (N_ret+N_int).T_DTU.
• Bounds on these parameters, i.e DelayMax, INPmin, and IAmin, provides full control of the retransmission scheme by the operator. In an embodiment, DelayMax provides a bound on the delay as for the interleaver. Note that the delayMax configured should be greater than the round trip delay. INPmin provides a bound on the impulse noise protection. IAmin provides the minimum guaranteed rate, R_in=(IAmin−INPmin.T_s)/IAmin.R_d.
• The particular end-to-end data transfer behavior in presence of retransmission needs new control parameters to provide guaranteed and predictable performances, both in term of available user data rate and jitter. In an exemplary embodiment, the following parameters may be used to this end and they are typically configurable independently per line and per direction:
• -maxDelay (expressed in ms)—This parameter (already used in case of normal interleaving) defines the maximum allowed nominal delay. It is used by the modem to set an upper bound on the allowed receiver retransmission queue size. Since it must be at least equal to the roundtrip delay, retransmission cannot be activated if the maxDelay configured is lower than 4 ms.
• -minimumRate (expressed in kb/s)—This parameter (already used in case of normal interleaving) defines the minimum guaranteed user data rate. The available bandwidth for retransmission is equal to the difference between the current data rate on the line and the minimum rate configured.
• -INPmin (expressed in 10th of symbol)—This parameter (already used in case of normal interleaving) defines the minimal guaranteed impulse noise protection, provided that the available data bandwidth allowed for retransmissions is not exceeded.
• -INPmax (expressed in 10th of symbol)—This parameter defines the maximum number of consecutive retransmissions that may take place and therefore bounds the maximal jitter due to retransmissions. A default value of zero doesn't bound the number of consecutive retransmissions (that will however never exceed maxDelay*4 symbols).
• -minRtxRatio (expressed in 1/256th, relative to the minimum data rate)—This parameter allows to provision a minimal guaranteed retransmission bandwidth, on top of the minimum rate. In case of repetitive impulses of known maximal length and periodicity, this parameter can be used to guarantee that the repetitive impulse noise can be corrected. A default value of zero doesn't force any extra guaranteed data bandwidth for retransmissions.
• -minRSoverhead (expressed in 1/256th)—This new parameter allows to forces a minimum amount of RS overhead (R/N). This can be used to guarantee a given amount of steady state error correction capability. A default value of zero doesn't force the use of RS overhead.
• In an embodiment using VDSL2, when the standard interleaving scheme is used, a common interleaving memory is dynamically split between an upstream (US) and (DS) directions. In a similar way, when the retransmission scheme is enabled, the retransmission memory is split between US and DS direction. In both cases, this split is performed in such way that the ratio of US and DS rate matches a given ratio configured independently for each line.
Performance for a Specific Embodiment
• Based on the previous model, the performances of an interleaver and a repetition scheme in a typical VDSL2 configuration were compared. A VDSL2 system with a symbol duration T_s=250 us, a roundtrip delay of 5 ms, an interleaver memory of 65536 bytes or a retransmission queue of 32768 bytes (assuming that the interleaver memory may be reused for retransmission) and a DTU duration T_DTU=260 us was used.
• The minimal number of consecutive retransmissions to achieve the INPmin is equal to
• $N_{\mathrm{ret}}=\lceil \frac{{\mathrm{INP}}_{\min }·\mathrm{Ts}}{T_{\mathrm{DTU}}}\rceil$
• The minimal number of DTU in the retransmission queue to handle both the roundtrip delay and the number of consecutive retransmissions is equal to:
• $W_{\mathrm{ret}}=\mathrm{MAX}\left(\lceil \frac{\mathrm{roundtrip}}{T_{\mathrm{DTU}}}\rceil ,N_{\mathrm{ret}}\right)$
• The maximal transmission rate of the DTU (equivalent to the bearer rate) is equal to:
• $R_d=\frac{\mathrm{Re}\mathrm{tFIFOSize}}{W_{\mathrm{ret}}·T_{\mathrm{TDU}}}$
• The ratio between the guaranteed transmission rate and the maximal transmission rate of the DTU
• $\frac{R_{\mathrm{in}}}{R_d}=\frac{N_{\mathrm{int}}}{N_{\mathrm{int}}+N_{\mathrm{ret}}}=1-\frac{N_{\mathrm{ret}}}{\lceil \frac{{\mathrm{IA}}_{\min }}{T_{\mathrm{DTU}}}\rceil }$
• Based on these formulae, the maximal transmission rate of the DTUs, the total delay, and the split between the delay incurred in the rate adaptation FIFO and the retransmission rescheduling queue, are derived for different INPmin in the table 1. The ratio between the guaranteed rate and the net rate is reported in table 2. As a comparison with an interleaver scheme, the maximal rate with the standard parameters of theprofile8awas derived for equivalent INPmin and MaxDelay. The results are illustrated, including the overhead of the Reed-Solomon in the table 3.

• TABLE 1
  Maximum bearer rate for different INPmin

  				Delay
  				rate-
  			Delay	adaptation	Total
  			Resequencing	FIFO	Delay	Max Rd
  INPmin	Nret	Wret	(ms)	(ms)	(ms)	(Mbps)

  1	1	20	5.2	0.26	5.46	50.4123
  2	2	20	5.2	0.52	5.72	50.413
  4	4	20	5.2	1.04	6.24	50.413
  8	8	20	5.2	2.08	7.28	50.413
  16	16	20	5.2	4.16	9.36	50.413

• TABLE 2
  Ratio guaranteed rate over net rate for different interarrival
  time and INP.

  minimum interarrival (ms)

  	ratio Rin/Rd	8	16	32	64	128

  		1	0.97	0.98	0.99	1.00	1.00
  	INPmin	2	0.93	0.97	0.98	0.99	1.00
  	(DMT)	4	0.87	0.93	0.97	0.98	0.99
  		8	0.73	0.87	0.93	0.97	0.98
  		16	0.47	0.74	0.87	0.93	0.97
  	NOTE:
  	Rdis the net data rate without retransmission and Rin is the guaranteed net data rate if impulse event of length INPmin DMT symbols arrives every inter-arrival time.

• TABLE 3
  Maximum net rate of an interleaver scheme with various
  MaxDelay and INPmin.

  INPmin	MaxDelay (ms)	Max Net rate (Mbps)	RS overhead (R/N)

  1	6	101.22	10.8%
  2	6	64.44	16.7%
  4	7	30.72	28.6%
  8	8	12.28	50%
  16	10	No solution	No solution

• By comparing these tables, it can be determined that, with equivalent delay, the interleaver performs better than the retransmission in terms of maximal achievable rate for low INP requirements (e.g. 1 or 2 DMT symbols). In embodiments, the retransmission scheme could have better performance if the roundtrip delay were decreased. Nevertheless, the interleaver scheme may suffer of a high Reed-Solomon overhead. This high overhead leads in general to a low or negative coding gain. Moreover, it should be carried even when the line is not subject to impulses while the retransmission scheme automatically increases the net data rate in good line conditions.
• In order to check the impact of low or negative coding gain, the performances of the interleaver scheme with the INPmin and DelayMax proposed in table 3 were compared with the performance of the retransmission mechanism where the reed-solomon overhead is near to optimum (e.g., RS(239,255)). Performances are depicted inFIG. 7, which illustrates a comparison of interleaver and retransmission schemes on realistic loop and impulse noise.
• The two curves shown with circles and squares show the respective performance of the proposed retransmission scheme with a maximal delay of 8 ms and an INPmin of 8 DMT symbols. In the first case, the inter-arrival time is set to 8 ms as it should be in case of REIN (REpetitive Impulse Noise) at 120 Hz. The performances of retransmission (square curve) outperform those obtained with the equivalent FEC scheme (triangle curve). In this embodiment, the retransmission provides around twice the INP than the FEC scheme for the same performance in case of REIN.
• In the second case, the performances were computed with an inter-arrival time of128ms as it may be set in a SHINE (Single High Impulse Noise Event) environment. In this embodiment, the performance was almost equal to the maximal performance achievable with the best coding gain, RDTU. This is an improvement than an FEC scheme where the amount of overhead is independent of the frequency of the worst case impulse.
Comparison Ideal Interleaver/Ideal Retransmission
• The previous section compared the interleaver and retransmission schemes with the standard limitations but it is possible to compare both schemes in a more general way. The formulas of table 4 are used to make the comparison. They are derived by assuming no limitations, like memory size, Dmax, size of DTU, and a perfect granularity in the setting of the interleaver and the retransmission. They can be seen as a continuous extension of the formula for real implementations. In this table:

• TABLE 4
  Formula for ideal retransmission and interleaver

  	Interleaver	Retransmission

  Delay (ms)	DelayMax	Roundtrip +
  		INPMin × Ts
  Reed-Solomonoverhead (R/N)	$\max \left(\frac{\mathrm{<figure-callout\; label="INPMin"\; filenames="US20080063007A1-20080313-D00007.png,US20080063007A1-20080313-D00011.png"\; state="\{\{state\}\}">INPMin</figure-callout>}}{2\times \mathrm{DelayMax}},\frac{16}{255}\right)$	$\frac{16}{255}$
  Guaranteed rate(kbps)	$\mathrm{Rline}·\left(1-\frac{R}{N}\right)$	$\begin{array}{c}\mathrm{Rline}·\left(1-\frac{R}{N}\right)·\\ \left(1-\frac{\mathrm{INPMin}\times T_s}{\mathrm{IAMin}}\right)\end{array}\hspace{1em}$
  Memory requirement(bits)	$\frac{\mathrm{DelayMax}\times \mathrm{<figure-callout\; label="Rline"\; filenames="US20080063007A1-20080313-D00007.png,US20080063007A1-20080313-D00011.png"\; state="\{\{state\}\}">Rline</figure-callout>}}{2}$	$\begin{array}{c}\mathrm{Roundtrip}\times \mathrm{Rline}\times \\ \left(1-\frac{R}{N}\right)\end{array}\hspace{1em}$
  NOTE
  The overhead of the retransmission has been selected to provide the coding gain of the concatenation of trellis and RS.
  Rlineis the data rate at the output of the RS encoder expressed in kbit/s.
  DelayMax is the maximum delay in ms.
  INPmin is the minimal INP in DMT symbol of duration Ts.
  R is the Reed-Solomon overhead and N is the Reed-Solomon codeword length.
  Roundtrip is the roundtrip in ms.
  IAmin is the minimum Interarrival Time in ms (as defined in section 4).

• From those equations, it is possible to compare the performance of the retransmission and interleaving scheme in terms of available guarantee data rate, delay, and memory usage for a given working point. A working point is defined by a set of MIB configuration of the INPmin, DelayMax, and IAmin; a certain loop and noise condition defined by Rline; and a roundtrip delay, Roundtrip. For all embodiments, the roundtrip delay was set to 5 ms.
• FIG. 8 is a comparison of guaranteed rate between retransmission and interleaver schemes.FIG. 8 plots the ratio between the guaranteed rates for retransmission and for interleaving in function of the DelayMax for different INPmin and IAmin. As illustrated, the performance is usually better with a retransmission scheme than with an interleaver when the retransmission may be used, i.e for a delayMax above (Roundtrip+INP_min.T_s) ms. Moreover, the advantage of the retransmission is increasing for higher INPmin value. These results are independent of R_line, so for a given loop and noise conditions, the retransmission will usually provide a better rate than the interleaver scheme. Note that IAmin=8 ms corresponds to a REIN-like configuration (IAmin is set equal to DelayMax when DelayMax is larger than IAmin).
• FIG. 9 illustrates the ratio between the delays for retransmission and for interleaving. This is independent of the IAmin. The retransmission has a total delay lower than the interleaver if the retransmission may be used. Moreover, as the delay of retransmission is independent of the maximum delay, the advantage of retransmission is increasing with an increase of DelayMax. These results are independent of R_line, so for a given loop and noise conditions, the retransmission will usually provide a lower delay than the interleaver scheme.
• FIG. 10 illustrates the memory required for the interleaver and retransmission normalized by R_line. This ratio is independent of the INPmin and IAmin. As illustrated, the retransmission used a fixed memory size while the interleaver memory increased with the delay. In fact above 9 ms of DelayMax, the retransmission used systematically less memory for an equal INPmin and R_line.
• FIGS. 7-10 derived in very general conditions show that, for most usual cases, retransmission provides better performance in terms of efficiency and delay than an interleaver scheme when retransmission may be used, i.e. when delayMax is larger than the roundtrip delay. This advantage becomes compelling for INPmin above 2. Furthermore, the retransmission usually requires less memory than the interleaver when the delay is above 9 ms.
• The embodiments described above may be adapted for use in existing systems using ADSL2 and VDSL2 standards, and may introduce retransmission in a transparent way. More specifically, certain embodiments may considerably limit the jitter introduced by a retransmission event; may be compatible with all type of data transported (ATM, PTM, or any type of encapsulation protocols); may minimize the modifications to existing systems; may be coupled to the physical link where the problem occurs; and may handle retransmission inside the constrains of a fixed data frame format that cannot cope with any data loss. In one variation, one may select the best place to insert the “retransmission layer” within an existing xDSL data flow.
Error Detection
• In embodiments, a retransmission scheme makes use of some kind of error detector. This error detector can be implemented in different ways and the exact implementation is unimportant considering only the feasibility of a retransmission scheme. Retransmission techniques make use of retransmission of the corrupted receive data in order to achieve the target BER requirement. One important operation is the identification of the corrupted data. This error detector has to be reliable in order to make the scheme efficient. In xDSL system, there are different types and locations for this error detector, including:
• • A specific detector based on some overhead added to the transmitted data
  • Make use of the already present CRC checksum
  • Make use of the already present RS overhead
• Reed-Solomon FEC redundancy combines the advantages of short round trip delay and availability in xDSL systems.
RS Based Error Detection
• In an embodiment, R bytes are used as Reed-Solomon overhead in a codeword of length N. The RS decoder can correct up to R/2 bytes in this codeword. Depending on the number of corrections made, one can compute the probability for a mis-correction, a correction which does do restore the original correct data. The probability is smaller as the number of corrections made to the codeword is smaller. For a given number of corrections, this probability is also smaller as the codeword length decreases.
• For embodiments of the disclosed retransmission scheme, the two following characteristics are desirable. First, the error detector should have some correction capability. For instance, assume that 5% of the codeword data are corrupted for every symbols (5% of the carrying tones suddenly very noisy for a long period), if there is no correction capability, the scheme would ask a retransmission for all the symbols leading rapidly to a frozen transmission. Second, the error detector should be highly reliable. As retransmission techniques allow to get rid of the data correction techniques or at least allow to reduce their correction capability, it is important that any corrupted data is detected as such in order to be retransmitted. Otherwise the corrupted data goes through with no or only limited possibility of further correction.
• The RS overhead is typically suited to cope with these two characteristics. Indeed, based on the chosen mis-correction probability p_mis-corr, the detector should correct the receive codeword if the mis-correction probability is smaller than p_mis-corr. In this case no retransmission is requested. Further, the detector should ask for a retransmission if the probability is larger than p_mis-corr. In this case, the received codeword will however be corrected in order to transfer the most likely codeword in case the retransmission does not arrive on time (before the codeword has to be transfer to upper layers).
SUMMARY
• The above described embodiments may be realized in hardware, software, or most commonly a combination thereof. Additionally, embodiments may be realized in a centralized fashion in at least one communication system, or in a distributed fashion where different elements may be spread across several interconnected communication systems. Any kind of computer system or other apparatus adapted for carrying out the methods described herein may be suited. A typical combination of hardware and software may be a general-purpose computer system with a computer program that, when being loaded and executed, may control the computer system such that it carries out the methods described herein.
• Alternatively, the above described embodiments may also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which when loaded in a computer system is able to carry out these methods. Computer program in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduction in a different material form.
• While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims.

Claims (14)

1. A apparatus for transmitting data in an xDSL system, comprising:

a transmitter that sends data over an xDSL system in the form of a data transmission unit (DTU), the transmitter having physical media specific—transmission convergence (PMS-TC) communication sublayer; and

a retransmission unit that is implemented in the PMS-TC sublayer, the retransmission unit including a retransmission buffer that stores and indexes transmitted DTUs in retransmit containers, the retransmit container being defined as a time slot corresponding to a sent DTU;

wherein the retransmission unit is configured to respond to a retransmission request indicating which stored DTU should be retransmitted, wherein the stored DTU to be retransmitted is identified by its corresponding retransmit container.

2. The apparatus ofclaim 1, wherein the DTU to be retransmitted is further identified by a unique sequence identification number (SID) that is added to the DTU in the PMS-TC sublayer.

3. The apparatus ofclaim 1, further including a forward error correction unit coupled to the retransmission buffer for encoding the DTUs.

4. The apparatus ofclaim 3, wherein the forward error correction unit uses Reed-Solomon encoding.

5. The apparatus ofclaim 3, wherein the DTU is defined as a group of n consecutive code words.

6. The apparatus ofclaim 1, further comprising a retransmission control channel coupled to the PMS-TC sublayer wherein the retransmission control channel is configured to receive the retransmission request from a receiver and retransmit the requested stored DTU.

7. A apparatus for transmitting data in an xDSL system, comprising:

a transmitter that sends data over an xDSL system in the form of a data transmission unit (DTU), the transmitter having a transport protocol specific transmission convergence (TPS-TC) communication sublayer and a physical media specific transmission convergence (PMS-TC) communication sublayer;

a retransmission layer disposed between the TPS-TC and PMS-TC communication sublayers; and

a retransmission unit that is implemented in the retransmission sublayer, the retransmission unit including a retransmission buffer that stores and indexes transmitted DTUs in retransmit containers, the retransmit container being defined as a time slot corresponding to a sent DTU;

wherein the retransmission unit is configured to respond to a retransmission request indicating which stored DTU should be retransmitted, wherein the stored DTU to be retransmitted is identified by its corresponding retransmit container.

8. The apparatus ofclaim 7, wherein the DTU to be retransmitted is further identified by a unique sequence identification number (SID) that is added to the DTU in the retransmission sublayer.

9. The apparatus ofclaim 7, wherein the DTU is defined as a fixed number of bytes from an asynchronous transfer mode (ATM) cell.

10. The apparatus ofclaim 7, wherein the DTU is defined as a fixed number of bytes from a packet transfer mode (PTM) fragment.

11. The apparatus ofclaim 7, further comprising a retransmission control channel coupled to the PMS-TC sublayer wherein the retransmission control channel is configured to receive the retransmission request from a receiver and retransmit the requested stored DTU.

12. A apparatus for transmitting data in an xDSL system, comprising:

a transmitter that sends data over an xDSL system in the form of a data transmission unit (DTU), the transmitter having a physical media specific transmission convergence (PMS-TC) communication sublayer and a physical media dependent (PMD) communication sublayer;

a retransmission layer disposed between the PMS-TC and PMD communication sublayers; and

a retransmission unit that is implemented in the retransmission sublayer, the retransmission unit including a retransmission buffer that stores and indexes transmitted DTUs in retransmit containers, the retransmit container being defined as a time slot corresponding to a sent DTU;

wherein the retransmission unit is configured to respond to a retransmission request indicating which stored DTU should be retransmitted, wherein the stored DTU to be retransmitted is identified by its corresponding retransmit container.

13. The apparatus ofclaim 12, wherein the DTU to be retransmitted is further identified by a unique sequence identification number (SID) that is added to the DTU in the retransmission sublayer.

14. The apparatus ofclaim 12, wherein the DTU is defined as a group of L bits mapped in a single DMT symbol.

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